1. Field of the Invention
The invention relates to a pneumatic brake booster with a booster housing which is subdivided into at least one working chamber and at least one vacuum chamber by means of at least one axially movable wall capable of being acted upon with a pneumatic differential pressure, with an actuable input member comprising a valve piston, with an output member for acting upon a brake master cylinder with an output force, with a control valve which is arranged in a control housing and is actuable by the valve piston and which controls the differential pressure, and with an elastic reaction element which is arranged in a control housing recess and against which the output member bears, means being provided which, with the brake booster assembled, make it possible to set a distance between the valve piston and the reaction element.
2. Description of the Related Art
The automobile industry lays down stringent requirements as regards the brake booster characteristic curve. It is therefore necessary to set the abovementioned distance between the valve piston and the reaction element, what is known as the z-dimension, as accurately as possible. As is known, the z-dimension determines the behavior of the brake booster in the initial phase of its actuation and makes it possible to have an abrupt rise in the output force or in the output pressure when the brake booster is actuated with a predefined input force (jumper effect). Owing to different shore hardnesses of the reaction element and to component tolerances, very high output force tolerances occur, and the output force lies within a very broad range (spread band). The requirements of the automobile industry as regards a very narrow spread band can therefore sometimes not be fulfilled.
A pneumatic brake booster of the generic type initially mentioned is known from DE 39 41 604 A1. In this, the setting of the z-dimension is carried out by changing the axial overall length of the valve piston of two-part design, in that a first piston part is rotated by means of a suitable tool with respect to a second piston part guided fixedly in terms of rotation in the control housing.
In another settable pneumatic brake booster which is known from DE 42 08 384 A1, the setting of the desired z-dimension is carried out by means of a sleeve which is permanently in engagement with the first piston part and is arranged coaxially with respect to the piston rod and which, after the setting operation has ended, is fixed in terms of rotation by means of its positive connection to the control housing. Fixing in terms of rotation in this case takes place preferably by means of a securing ring which is plugged onto the sleeve and the radial projections of which engage into grooves formed on the inner circumference of the control housing. The securing ring is subsequently held by the marginal region of an elastic protective cap protecting the control housing, said marginal region being buttoned in a circumferential groove formed in the sleeve.
The complicated set-up of the known brake booster with a settable z-dimension is considered to be a disadvantage. Furthermore, the connection of the input member to a brake pedal, said connection having to be released during setting, is a disadvantage.